J. Meteor. Res.  2018, Vol. 32 Issue (3): 380-393 PDF
http://dx.doi.org/10.1007/s13351-018-7046-5
The Chinese Meteorological Society
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#### Article Information

YAN, Xin, Juzhang REN, Jianhua JU, et al., 2018.
Influence of Springtime Atlantic SST on ENSO: Role of the Madden–Julian Oscillation. 2018.
J. Meteor. Res., 32(3): 380-393
http://dx.doi.org/10.1007/s13351-018-7046-5

### Article History

Received April 24, 2017
in final form January 18, 2018
Influence of Springtime Atlantic SST on ENSO: Role of the Madden–Julian Oscillation
Xin YAN1,2, Juzhang REN3, Jianhua JU4, Song YANG1,2
1. School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275;
2. Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou 510275;
3. Yunnan Research Institute of Meteorology, Kunming 650034;
4. Yunnan Provincial Meteorological Bureau, Kunming 650034
ABSTRACT: Increased evidence has shown the important role of Atlantic sea surface temperature (SST) in modulating the El Niño–Southern Oscillation (ENSO). Persistent anomalies of summer Madden–Julian Oscillation (MJO) act to link the Atlantic SST anomalies (SSTAs) to ENSO. The Atlantic SSTAs are strongly correlated with the persistent anomalies of summer MJO, and possibly affect MJO in two major ways. One is that an anomalous cyclonic (anticyclonic) circulation appears over the tropical Atlantic Ocean associated with positive (negative) SSTA in spring, and it intensifies (weakens) the Walker circulation. Equatorial updraft anomaly then appears over the Indian Ocean and the eastern Pacific Ocean, intensifying MJO activity over these regions. The other involves a high pressure (low pressure) anomaly associated with the North Atlantic SSTA tripole pattern that is transmitted to the mid- and low-latitudes by a circumglobal teleconnection pattern, leading to strong (weak) convective activity of MJO over the Indian Ocean. The above results offer new viewpoints about the process from springtime Atlantic SSTA signals to summertime atmospheric oscillation, and then to the MJO of tropical atmosphere affecting wintertime Pacific ENSO events, which connects different oceans.
Key words: Madden–Julian Oscillation (MJO)     Atlantic, sea surface temperature anomaly (SSTA)     Walker circulation     teleconnection     El Niño–Southern Oscillation (ENSO)
1 Introduction

The relationship between the Atlantic sea surface temperature (SST) and El Niño–Southern Oscillation (ENSO) has been investigated previously. Substantial effort has been devoted to understanding the influence of ENSO events on the Atlantic SST. For example, analyses suggested that Atlantic warming occurs 4–6 months after the mature phase of a Pacific warm event (Wright, 1986; Enfield and Mayer, 1997; Penland and Matrosova, 1998; Klein et al., 1999; Giannini et al., 2000; Hu et al., 2016). The remote influence of ENSO on the climate over other parts of the world has also been studied extensively (e.g., Chang et al., 1997; Uvo et al., 1998; Saravanan and Chang, 2000; Chiang and Sobel, 2002). Mo and Häkkinen (2001) indicated that the SST anomalies in the northern and southern tropical Atlantic Ocean were associated with two different frequency components of ENSO variability. Huang (2004) discussed the role of wave train and sea level pressure in the transmission of ENSO signals to the Atlantic SST. Studies (e.g., Chang et al., 2006) further showed that the net effect of El Niño on the Atlantic Niño depended not only on the atmospheric response that transmitted El Niño signal to the tropical Atlantic, but also on the dynamic ocean–atmosphere interaction in the equatorial Atlantic that worked against the atmospheric response.

Some studies focused on the influence of Atlantic SST on equatorial Pacific SST (Dommenget et al., 2006; Jansen et al., 2009; Rodríguez-Fonseca et al., 2009; Ding et al., 2012; Frauen and Dommenget, 2012; Martín-Rey et al., 2012). The study using reanalysis data and atmospheric circulation model by Ham et al. (2013a) indicated that the springtime northern tropical Atlantic SST might be a trigger of ENSO events. Ham et al. (2013b) illustrated that the northern tropical Atlantic SST and the Atlantic Niño were likely linked to different types of El Niño events. Similar results were shown in Ding et al. (2017). Polo et al. (2015) reported a mechanism that warm (cool) SST anomalies in the equatorial Atlantic in summer were related to ascending (descending) motions over the Atlantic and subsidence over the Pacific. Martín-Rey et al. (2014) found that the Atlantic–Pacific Niño’s connection was modulated at multidecadal timescales coinciding with the negative phase of the Atlantic Meridional Oscillation (AMO), which was supported by climate model results (Sasaki et al., 2014; Ham and Kug, 2015).

However, the physical mechanisms for the above findings have not been fully understood despite the influence of cyclonic circulation which helps to explain how the Atlantic SST anomaly affects the occurrence of Pacific ENSO events. Moreover, details of the long-term air–sea interaction from spring to winter are unclear. In the current study, we focus on the relationship between the Atlantic SST and the Madden–Julian Oscillation (MJO), emphasizing the role of the North Atlantic Oscillation (NAO) in the variability of the tropical atmosphere.

A number of studies examined the relationship be-tween ENSO and MJO, which is characterized by two different dominant timescales (Lau and Waliser, 2011; Li, 2014; Li et al., 2014). It was shown that this relationship is due to the response of the ocean to MJO activity (Zavala-Garay et al., 2005; Tang and Yu, 2008a; Zavala-Garay et al., 2008; Seiki et al., 2009; Hoell et al., 2014). Gushchina and Dewitte (2012) found that tropical intraseasonal variability could be more favorable for triggering eastern Pacific El Niño events than for triggering central Pacific El Niño events. Some studies also examined the difference in MJO activity between central Paciﬁc (CP) and East Paciﬁc (EP) El Niño cases (Hoell et al., 2014; Chen et al., 2015; Feng et al., 2015). Yan and Ju (2016) examined the persistent anomalies of summer MJO; and Yan et al. (2016) indicated that these persistent anomalies were closely related to winter ENSO events. In addition, MJO modulates the intensity and frequency of westerly wind events over the western Pacific warm waters. However, the methods used to define MJO in different studies may affect the results obtained. For example, the disagreement between Seiki and Takayabu (2007) and Chiodi et al. (2014) in explaining the relationship between the westerly wind events and MJO can be attributed to the different methods used to define MJO events. While Seiki and Takayabu (2007) used an index for the vicinity of the westerly wind events to characterize the state of MJO, the Wheeler and Hendon (2004) index used by Chiodi et al. (2014) was meant to capture MJO activity at the global scale and may not accurately depict the local MJO signals over the western Pacific. It should be noted that in the studies related to ENSO, the use of different definition methods for the MJO index may also result in different results. We conduct this study to explore the possible reasons for the persistent anomalies of summer MJO and to understand the precursory signals of these persistent anomalies. In particular, we discuss the physical process of springtime Atlantic SST anomalies that affect wintertime ENSO events.

2 Data and method 2.1 Data

Multiple datasets are used in this study, including (1) the NCEP/NCAR reanalysis data (Kalnay et al., 1996), (2) the NOAA Extended Reconstructed Sea Surface Temperature (ERSST) data (Xue et al., 2003; Smith et al., 2008), (3) the MJO indices defined by the NOAA Climate Prediction Center (CPC) (Xue et al., 2002), and (4) the oceanic Niño index (ONI) provided by the CPC. As shown inTable 1, the MJO indices represent the active and inactive conditions of tropical MJO at different longitudes. The ONI was obtained by using a 3-month moving average of SST in the Niño3.4 area (5°S–5°N, 120°–170°W) at http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears_ERSSTv3b.shtml.

Table 1 Longitudes corresponding to the 10 MJO indices over the equator
 MJO index Longitude MJO1 80°E MJO2 100°E MJO3 120°E MJO4 140°E MJO5 160°E MJO6 120°W MJO7 40°W MJO8 10°W MJO9 20°W MJO10 70°W
2.2 Definition of the IIP index

Yan and Ju (2016) found that the stagnant centers of MJO action existed in summer when its eastward propagation became weaker. Figure 1 shows three different cases of MJO propagation. The usual MJO propagation, as in summer 2001 (Fig. 1a), extends from west to east without a pause. If we focus on the equatorial In-dian Ocean (70°–80°E), there are two complete MJO life cycles from 1 June to 31 August. As is well known, there are two active regions of MJO: one is over the Indian Ocean, and the other is over the equational Pacific Ocean. Because the CPC MJO indices are obtained with an extended empirical orthogonal function, there are also two MJO life cycles over the equational Pacific Ocean (160°E–120°W). But the propagation of MJO is not always like that in summer 2001. One typical unusual condition is shown in Fig. 1b: during summer 1982, the MJO was more active over the equatorial Pacific Ocean than over the equatorial Indian Ocean, while the eastward propagation feature of MJO is not significant. The stagnant center of MJO was over the equatorial Pacific Ocean in this case. Another unusual condition is shown in Fig. 1c, in which the MJO was more active over the Indian Ocean during summer 1988. In this case, the stagnant center of MJO was over the equatorial Indian Ocean while the eastward propagation of MJO was also not apparent.

 Figure 1 CPC’s MJO indices during May–September of (a) 2001, (b) 1982, and (c) 1988.

The examples shown above suggest that the stagnant centers of MJO can be present as two persistent and anomalously active centers over the equatorial Pacific and the equatorial Indian Ocean, respectively. To represent the persistent anomalies of MJO in summer, an IIP index is defined as follows:

 ${{{I}}_{{\rm{IP}}}}{\rm{ = }}\frac{{\rm{1}}}{{\rm{3}}}{\sum\nolimits_{{{i = \rm{6}}}}^{\rm{8}} {\left[ {\left({\rm{MJO10 + MJO1}} \right) - \left({\rm{MJO5 + MJO6}} \right)} \right]} _i}.$ (1)

In Eq. (1), i = 6, 7, 8 refers to June, July, and August, respectively, and the IIP index indicates a three-month average. Four of the 10 CPC MJO indices are applied here (MJO10, MJO1, MJO5, and MJO6) to calculate the IIP index, which represents the stagnant intensity of MJO. The IIP index indicates three kinds of possible summer MJO features. (1) When the IIP value is close to zero, the index indicates that the summer MJO extends normally from west to east without a pause, or the summer MJO is very weak. Like the case in 2001 (Fig. 1a), the IIP value was –0.66, while the summer MJO extended normally with a clear eastward trajectory. (2) If the IIP value is positive, the summer MJO is more active over the Pacific than over the Indian Ocean. When the IIP is particularly large, the summer MJO is significantly active. Like the 1982 case (Fig. 1b), the IIP value was 3.98, while the MJO exhibited significant persistent anomalies over the Pacific Ocean. (3) If the IIP value is negative, the summer MJO is more active over the Indian Ocean than over the Pacific Ocean. As in the 1988 case (Fig. 1c), the IIP value was –4.98, while the summer MJO showed significant persistent anomalies over the Indian Ocean.

In this study, two kinds of typical years are selected. First, IIP was greater than 1 (including 1982, 1986, 1987, 1990, 1991, 1997, 2002, and 2009), when persistent anomalies of summer MJO appeared over the Pacific Ocean. Second, IIP was less than –1 (including 1988, 1995, 1996, 1998, 1999, 2007, 2008, and 2010), when persistent anomalies of summer MJO were seen over the Indian Ocean.

3 Spring Atlantic SST and persistent anomalies of summer MJO

To search for possible factors that influence the persistent anomalies of summer MJO, we analyze the variability of winter and spring Atlantic SSTs. The correlation between Atlantic SST anomaly (SSTA) and IIP (Fig. 2) indicates that warmer (colder) spring Atlantic SST is highly related to persistent anomalies of summer MJO over the Indian (Pacific) Ocean. The results show that negative SSTAs appear over the southern tropical Atlantic Ocean. In North Atlantic, the distribution of SSTA resembles a tripole pattern. Previous studies have shown that the North Atlantic tripole SSTA pattern affects variability of the East Asian summer monsoon in its early stage (Gu et al., 2009; Wu et al., 2012; Zuo et al., 2013). Warmer North Atlantic SST may be regarded as a “trigger” for ENSO events (also see Martín-Rey et al., 2012). Here, we define two domains where SST variability may be important (see Table 2): the South Atlantic and equatorial Atlantic region as key area A1 and the North Atlantic region as key area Atri. Atri comprises three sub areas: Atri-1, Atri-2, and Atri-3. The locations of these areas are outlined in Fig. 2 and specified in Table 2.

 Figure 2 Correlation between springtime Atlantic SSTA and the IIP index of subsequent summer during 1979–2013. Shadings indicate the values that statistically exceed the 90% confidence level. The boxes denote the key areas A1 (red dashed), Atri-1 (green dashed), Atri-2 (red solid), and Atri-3 (yellow dashed).
Table 2 Key areas used to define springtime North Atlantic SSTA
 Key area A1 22°S–10°N, 40°–0°W Atri-1 42°N–52°N, 40°–20°W Atri-2 28°N–38°N, 80°–60°W Atri-3 18°N–28°N, 60°–50°W

The SSTA of the North Atlantic tripole pattern, i.e., ${\Delta T_{{\rm{A}}_{\rm{tri}}}}$ , is defined as follows:

 ${ \Delta T_{{\rm{A}}_{\rm{tri}}}} = \left({{\Delta T_{{\rm{A}}_{{\rm{tri}}{\text{-}}1}}} - {\Delta T_{{\rm{A}}_{{\rm{tri}}{\text{-}}2}}} + {\Delta T_{{\rm{A}}_{{\rm{tri}}{\text{-}}3}}}} \right)/3.$ (2)

In consideration of the SSTAs in A1 (tropical South Atlantic) and in Atri (i.e., the North Atlantic tripole pattern), we define an IATL index of the Atlantic SSTAs below to describe the overall effect of Atlantic SST abnormality,

 ${I_{\rm{ATL}}} = \left({{{\Delta T_{\rm{A1}}}} + {\Delta T_{{\rm{A}}_{\rm{tri}}}}} \right)/2.$ (3)

Figure 3 shows the year-by-year sequence of IATL index in spring during 1979–2013, and compares it with the sequence of the IIP index in summer. The correlation coefficient between IATL and IIP is –0.74. Following the approach of defining IIP typical years in Section 2, 8 years are selected as high IATL years (1988, 1995, 1998, 2001, 2005, 2007, 2008, and 2010), and 8 years are selected as low IATL years (1982, 1985, 1986, 1989, 1992, 1997, 2002, and 2012).

 Figure 3 Time series of the IATL index (red curve) of springtime Atlantic SSTA and the IIP index (blue curve) of persistent anomaly of summer MJO. Since the IATL index is negatively correlated with IIP, IIP is multiplied by –1 here.

Given the high correlation discussed above, the years with higher (lower) spring SST in the Atlantic key areas correspond to the years with persistent anomalies of active summer MJO over the Indian (Pacific) Ocean. The composite MJO index values in the typical years of IATL and IIP indicate that the MJO presents persistent anomalies in summer (Fig. 4a) over the Indian Ocean. In the meantime, the MJO is restrained over the Pacific region. This feature is similar to the persistent anomalies of MJO in the typical year with a low IIP value (Fig. 4b). In the summer of the year with a low IATL value (Fig. 4c), MJO anomaly is active over the Pacific but is restrained over the Indian Ocean. This feature is similar to the persistent anomalies of MJO in the typical year with a high IIP value (Fig. 4d). It is thus clear that the anomalies of spring Atlantic SST may result in persistent anomalies of summer MJO.

 Figure 4 Composite MJO indices of CPC for (a) high IATL years, (b) low IIP years, (c) low IATL years, and (d) high IIP years. Shadings indicate statistically significant correlation above the 90%, 95%, 98%, 99%, and 99.9% confidence level, respectively. The x-axis shows longitude and the y-axis shows time of January (1) to December (12).

The above analysis indicates that spring Atlantic SST anomalies are significantly correlated with the persistent anomalies of summer MJO. Since the spring Atlantic SST anomalies defined by IATL can be regarded as a precursory signal of the persistent anomalies of summer MJO, we will discuss the physical mechanism involved in the next section.

4 Mechanism for the possible impact of spring Atlantic SSTA on persistent anomalies of summer MJO 4.1 Variation in atmospheric circulation related to spring Atlantic SSTA

To investigate the specific processes through which the spring Atlantic SSTA influences MJO activity over the tropical Indian Ocean and tropical Pacific Ocean, we examined the circulation difference between typical high and low IATL years (see discussion of Fig. 3 in Section 3). The composite differences of 850-hPa wind field in spring and summer are shown in Fig. 5. In spring (Fig. 5a), corresponding to the positive SSTA in North Atlantic, a notable anomalous cyclone is located over the North Atlantic Ocean. As a Gill-type Rossby wave response (Matsuno, 1966; Gill, 1980), the anomalous cyclone enhances the westerly anomalies over the tropical Atlantic, and there appear significant westerly anomalies over the eastern tropical Pacific. Meanwhile, an anomalous anticyclonic circulation occurs over the western subtropical Pacific, which enhances the easterly anomalies over the western tropical Pacific. Because of the easterly and westerly anomalies over the equatorial Pacific, divergence occurs over 180°–150°W. With the development of such a circulation into summer (Fig. 5b), both the easterly over the western tropical Pacific and the westerly over the eastern tropical Pacific and Atlantic are enhanced in the lower troposphere.

 Figure 5 Composite differences of horizontal wind (vector; m s–1) and vertical wind (shading; m s–1; updraft in red) at 850 hPa between the high and low IATL years in (a) spring and (b) summer. The vertical velocity is enhanced by –100 times. Only regions that statistically exceed the 0.1 significance level according to the Student’s t-test are plotted.

The upper troposphere shows opposite circulation features. Easterly anomalies prevail over the tropical Atlantic and eastern Pacific in spring (Fig. 6a), and an anticyclone appears to their north. The tropical easterly anomalies strengthen in summer (Fig. 6b); in the meantime, westerly anomalies prevail over the tropical Pacific, accompanied by a pair of cyclonic patterns on both sides of the equator. Convergence occurs in the middle of easterly and westerly anomalies over the tropical Pacific.

 Figure 6 As in Fig. 5, but at 200 hPa.

As can be seen from vertical wind differences at 850 and 200 hPa, updraft occurs over the eastern tropical Pacific and the Atlantic and downdraft occurs over the western tropical Pacific in spring (Figs. 5a, 6a). The circulation pattern is enhanced in summer, and another downdraft occurs over the equatorial Indian Ocean (Figs. 5b, 6b). To better understand the characteristics of verti-cal motion, Fig. 7 is plotted to show composite differences of equatorial vertical velocity between typical high and low IATL years. In spring, significant updraft occurs at 80°W, corresponding to the positive tropical Atlantic SSTA; and the maximum updraft appears between 600 and 800 hPa (Fig. 7a). In addition to the lower tropospheric westerly anomaly (Fig. 5a) and the upper tropospheric easterly anomaly (Fig. 6a) over the Atlantic, an anomalous vertical-zonal cell appears from the eastern tropical Pacific to the Atlantic Ocean. It acts in spring as a secondary circulation to the Walker circulation, favoring development of the Pacific Walker circulation. Previous studies (Wang et al., 2010; Kucharski et al., 2011; Frauen and Dommenget, 2012) showed that the Walker-type and Hadley-type circulations in the Western Hemisphere caused by Atlantic SSTAs could affect tropical general circulation and contribute to the generation of ENSO events. Thus, with the development of such a circulation into summer, enhancement of the anomalous downdraft branch of the zonal circulation over 180°–150°W (Fig. 7b) leads to upper tropospheric convergence and sinking motion between the westerly wind anomaly over the central tropical Pacific and the easterly wind anomaly over the tropical Atlantic (Fig. 6b). Accordingly, in the lower troposphere, both the easterlies over the tropical Pacific Ocean and the westerlies over the tropical Atlantic are enhanced (Fig. 5b), and divergence occurs over the central tropical Pacific. Meanwhile, another anomalous updraft branch of the Walker circulation at 120°E strengthens (Fig. 7b), so that an active tropical convergence belt appears from the tropical Indian Ocean to the western Pacific Ocean. Accordingly, convective activity and continuous summer MJO activity occur frequently over the Indian Ocean.

 Figure 7 Composite differences of vertical wind (shading; Pascal s–1; updraft in red) between the high and low IATL years over the equator in (a) spring and (b) summer. The vertical velocity is enhanced by –100 times. Stippled regions are statistically significant, exceeding the 0.1 significance level, according to the Student’s t-test.

Based on the above analysis of the circulation evolution, we propose a possible pathway via which the Atlantic SSTA in spring may give rise to persistent summer MJO anomalies (see Fig. 8). In high IATL years, as a Gill-type response to the positive Atlantic SSTA, a pair of cyclones occur in the lower troposphere over the subtropical Atlantic. Meanwhile, updraft occurs over the tropical Atlantic (Fig. 8a). On the other hand, an anticyclonic circulation appears in the lower troposphere over the central subtropical Pacific. As a result, both the westerly anomaly in the lower troposphere over the tropical Atlantic and the easterly anomaly over the central tropi-cal Pacific intensify. Lower tropospheric westerlies and upper tropospheric easterlies occur over the eastern tropical Pacific–tropical Atlantic regions. A secondary circulation forms over the equatorial eastern Pacific–Atlantic region in summer, so that the downdraft branch of the Walker circulation over the eastern Pacific is streng-thened, further intensifying the Walker circulation. Moreover, the tropical convergence belt is active over the Indian Ocean and the western Pacific warm pool; accordingly, convective activity is frequent. In the end, this leads to continuous activity of summer MJO over the Indian Ocean. In low IATL years (Fig. 8b), the changes in SST, upper and lower tropospheric circulations including the MJO are mirrors to those in high IATL years. On the whole, positive (negative) spring equatorial Atlantic SSTA results in updraft (downdraft) that drives the secondary circulation on the east branch of the Walker circulation, so that the equatorial Walker circulation is strengthened (weakened) in summer. Finally, continuous active anomalies of summer MJO are present over the Indian Ocean or the Pacific Ocean.

 Figure 8 Schematic diagram showing MJO–Atlantic SST interaction in (a) high IATL years and (b) low IATL years, under combined effect of the Walker circulation and the secondary circulation.
4.2 The circomglobal teleconnection and the Indian Ocean relaying effect

The North Atlantic tripole SSTA may enhance atmospheric teleconnection patterns and favor eastward transmission of the AO and NAO signals from the mid–high latitudes to the low latitudes, according to previous studies on the relationship between winter–spring North Atlantic tripole SSTA and the Asian summer monsoon (Gu et al., 2009; Wu et al., 2012; Zuo et al., 2013). Ding and Wang (2005) defined a circumglobal teleconnection (CGT) pattern, in which the circulation anomalies over the North Atlantic and European regions lead to atmospheric circulation anomalies in the Asian subtropical jet stream, which generates a teleconnection pattern from Pakistan to North America via East Asia and North Pacific. In the spring of high IATL years (Fig. 9a), a strong high pressure anomaly center exists over the extratropi-cal North Atlantic, and low pressure anomaly centers appear over the southwest of the East Europe Plain and the Mediterranean Sea–Arabian Peninsula regions. The high–low–high pressure pattern, similar to the second scenario in the CGT pattern noted inDing and Wang (2005), can transmit energy from the mid–high latitudes of North Atlantic to the low latitudes of Eurasia, causing convective activity over the Indian Ocean. In the spring of low IATL years (Fig. 9b), on the contrary, a strong low pressure anomaly center exists over the extratropical North Atlantic, a high pressure anomaly center occurs over the south-west of the East Europe Plain, and another low pressure anomaly center appears over the Mediterranean–Arabian Peninsula regions. This teleconnection pattern transmits energy from North Atlantic to the mid–low latitudes of Eurasia, restraining the occurrence of convective activity over the Indian Ocean.

 Figure 9 Composite geopotential height anomaly (gpm) at 500 hPa in spring for (a) high IATL years and (b) low IATL years. The stippled areas denote the anomalies that are significant at the 90% confidence level.

Another possible mechanism of spring Atlantic SSTA leading to the persistent summer MJO anomalies is the Indian Ocean relaying effect. Figure 10 shows regression analysis of the outgoing longwave radiation (OLR) and SST in summer against the IATL time series, which illustrates the late response of the atmosphere and ocean generated by the forced Atlantic SSTA in spring. A remarkable feature is that the tropical Indian Ocean has a large positive SSTA region with negative anomalous OLR above it. Correspondingly, positive OLR anoma-lies occur above the negative SSTAs in the central equatorial Pacific. This is consistent with the finding of Yu et al. (2016), who illustrated a northern Indian Ocean relaying mechanism through which the tropical Atlantic SSTA can remotely influence the summer monsoon mean circulation in the western North Pacific.

 Figure 10 Regressed SST (shading; only regions statistically exceeding the 0.1 significance level are plotted) and OLR (contour areas exceeding the 0.01 significance level are dotted) in summer against the IATL.
5 Processes involved in spring Atlantic SSTA leading to ENSO events

The relationship between antecedent Atlantic SST and later Pacific SST has been studied extensively (e.g., Alexander et al., 2002; Timmermann et al., 2007). Ham et al. (2013a) divided the Atlantic SST into two categories: cold tropical North Atlantic SST and Atlantic Niño, which correspond to different ENSO events. Martín-Rey et al. (2012) also considered spring Atlantic SSTA as a trigger of ENSO events. These studies have established a link between spring Atlantic SSTA and the following winter Pacific SSTA. Efforts have also been devoted to understanding the relationship between MJO and ENSO (Kessler and Kleeman, 2000; Bergman et al., 2001; Kessler, 2001; Zhang and Gottschalck, 2002; Seo and Xue, 2005; McPhaden et al., 2006; Hendon et al., 2007; Lin and Li, 2008; Tang and Yu, 2008a, b; Newman et al., 2009). Although we have proposed a possible pathway for the effect of spring Atlantic SSTA on the persistent anomalies of summer MJO, a question remains: Do the persistent anomalies of summer MJO act as a link between the spring Atlantic SSTA and the autumn–winter ENSO events?

Figure 11 shows the year-by-year variations of the IATL index of spring Atlantic SSTA, the IIP index of persistent summer MJO anomalies, and the ONI index of the following winter ENSO during 1979–2013. It can be seen that IIP exhibits a close relationship with ONI, while IATL shows reverse relationships with IIP and ONI. These results demonstrate a link of the key signals in spring, summer, and winter. A possible air–sea interaction pathway can be proposed for the spring Atlantic SSTA, persistent summer MJO anomalies, and winter ENSO events, based on analysis of the following features (summarized in Fig. 12).

(1) The correlation between IATL of spring Atlantic SSTA and IIP of persistent summer MJO anomalies is highly significant, with a correlation coefficient of –0.74. The spring Atlantic SSTA leads to active persistent MJO anomalies over the Pacific or the Indian Ocean. An ano-malous secondary circulation appears east of the Walker circulation and a lower tropospheric cyclone (anti-cyclone) appears over the Atlantic, which strengthens (weakens) the Walker circulation. A high (low) pressure anomaly occurs over North Atlantic and affects the convective activity over the tropical Indian (Pacific) Ocean. (2) The summer IIP and the winter ONI are highly corre-lated, with a correlation coefficient of 0.89. Yan et al. (2016) found that there exists a close relationship bet-ween persistent anomalies of summer MJO and autumn–winter ENSO events. This is because the persistent summer MJO anomalies cause wind stress anomalies over the equatorial Pacific, and the warm or cold ocean Kelvin wave caused by air–sea interaction produces warm or cold sea water in the sub surface of the equatorial Pacific, which is piled up to the east of the Pacific Ocean, activating and promoting ENSO events. (3) Given the significant correlation between spring IATL and winter ONI (R = –0.66), we propose a possible pathway for the interaction between ocean and atmosphere, in which the spring Atlantic SSTA affects the development of the autumn–winter ENSO events. The positive (negative) spring tropical Atlantic SSTA enhances (weakens) the secondary circulation of the Walker circulation. In addition, the anomaly of high–low pressure over North Atlantic is transmitted to the mid–low latitudes of the Indian Ocean through the CGT and/or the Indian Ocean relaying effect. Thus, the spring Atlantic SSTA leads to persistent summer MJO anomalies over the Indian/Pacific Ocean. The inactive/active anomalies of summer MJO over the western Pacific further weaken/enhance the Walker circulation over the Pacific, so that significant easterly/westerly anomalies occur in the lower troposphere causing westward/eastward wind stress anomalies, leading to rever-sed temperature changes in the sub surface of the Pacific Ocean. Finally, the SSTAs in the eastern equatorial Pacific are strengthened, altering the intensity of winter ENSO events.

 Figure 11 Time series of the IATL index in spring (red), the IIP index in summer (blue), and the ONI index in winter (black).
 Figure 12 Schematic diagram showing the relationship of the Atlantic SSTA in spring, the MJO persistent anomalies in summer, and the ENSO events in winter.
6 Conclusions and discussion

In this paper, we have discussed the relationship between spring Atlantic SSTA and persistent summer MJO anomalies. We infer that warmer spring Atlantic SST could lead to persistent summer MJO anomalies over the Indian Ocean, while colder spring Atlantic SST tends to cause persistent summer MJO anomalies over the Pacific Ocean. There are two possible pathways for the spring Atlantic SSTA leading to the persistent summer MJO anomalies. One involves the cyclone/anticyclone caused by the Atlantic Ocean SSTA, which affects the equatorial vertical circulation. This circulation pattern leads to strengthening or weakening of the Walker circulation in summer, so that anomalous updraft occurs over the equatorial region of the Indian Ocean or the eastern Pacific. The other pathway involves the high/low pressure anomaly caused by the North Atlantic triple SSTA pattern, which is transmitted to the mid–low latitudes by the circumglobal teleconnection pattern, affecting the convective activity over the Indian Ocean. The Indian Ocean relaying effect is also a possible mechanism for the spring Atlantic SSTA influencing the persistent summer MJO anomalies.

Given the increasing number of studies on the role of Atlantic SST in triggering ENSO events, we speculate whether the MJO may be one of the pathways to connect the two oceans. Based on the evidence that MJO can affect the occurrence of ENSO events (Yan et al., 2016), we infer that the persistent summer MJO anomalies may be a possible way. Since MJO is an important system in the tropical atmosphere that links the Atlantic and Pacific oceans, we raise a presumption that oceanic signal is transmitted into the atmosphere and then back to the ocean, in this paper. In this air–sea interaction mechanism, the signal of persistent summer MJO anomalies or even spring Atlantic SSTA could be regarded as an important indicator for the occurrence of winter ENSO events.

In this study, we explored the role of persistent summer MJO anomalies in connecting spring Atlantic SSTA to winter ENSO events. It should be pointed out that the persistent summer MJO anomalies are not necessarily a typical MJO, but may be more similar to a local MJO signal. Due to different characteristics of MJO propagation, previous studies focused on different definitions of MJO indices (Kikuchi et al., 2012; Kiladis et al., 2013). Several studies discussed the contradiction between different findings due to different definitions of MJO indices. For example, Puy et al. (2016) pointed out that the reason for the contradictions was related to whether the MJO activity was at local or global scales. The MJO persistence signal associated with ENSO events discussed in Yan et al. (2016) was also not typical for 30–60-day oscillations, but a kind of forced local oscillations obser-ved in MJO signals. Such signals are manifested in the current MJO monitoring index used at the NOAA CPC and the real-time multivariate MJO indices. In contrast, Kiladis et al. (2013) defined an alternate filtered MJO OLR (FMO) index obtained by filtering OLR, which is similar to the real-time multivariate MJO indices; however, the FMO index did not exhibit ENSO signals based on our calculation.

Nevertheless, Gushchina and Dewitte (2012) indicated that the intraseasonal variability of the atmosphere was associated with different types of El Niño, while Ham et al. (2013b) believed that different signals of the early Atlantic SSTA were also related to different types of El Niño. Furthermore, studies began to discuss the diversity of La Niña in the current classification of ENSO events (Kug and Ham, 2011; Ren and Jin, 2011; Shinoda et al., 2011). In particular, the indices defined in recent studies (Hu et al., 2016; Wiedermann et al., 2016) clearly distinguish the eastern Pacific (EP) El Niño, central Pacific (CP) El Niño, EP La Niña, and CP La Niña events. Therefore, analysis of different types of MJO and their impacts on different types of ENSO events is needed in future studies.

Acknowledgments. The authors thank the constructive suggestions from the two anonymous reviewers, from Dr. Junmei Lyu of the Chinese Academy of Meteorological Sciences, and from Professor V. Krishnamurthy of the Geroge Mason University.

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